Production and separation of anhydrous alkali metal sulfate and boric acid



D. S. TAYLOR AND SE PRODUCTION PARATION oF ANHYDRoUs ALKALI METAL SULFATE AND BORIC ACID Filgd July 51, 1947 May 5, 1953 .xomwx \A 7 W Md @n F kw .w u) ,was W7.. r Mx/ m 9G60 V U m07 OBN`QXO`X 3 u n A f w Mm. fo n ,W5 8% oooooeo M E L -r LWN l o C /o/ ,W w .J 4 oboe M .d DGG-0O@ M C M n m z 3 fw, y 44 A@ o n u 4 z wflpw; o 04 Y V .n64 1 lvll y .JWM 4 L pr eooeoo/J f y; Lb i @wwf l wf. wwf J il 5 7. e s r c s f Th d. w 4 a S 0 l) n 0 w ,JW 2 e 5 2 b o 0 C oz J Ww .m7, .o N

Patented May 5, 1953 PRODUCTION AND SEPARATION F ANHY- DROUS ALKALI METAL SULFATE AND BORIC ACID Donald S. Taylor, Monrovia, Calif., assignor, by mesne assignments, to Boi-ax Consolidated, Limited, London, England, a corporation of Great Britain and Northern Ireland Application July 31, 1947, Serial No. 765,095

Claims. l

This invention is concerned generally with stabilized supersaturation in certain types of chemical solutions, and, more particularly, with methods of controlling such supersaturation. Although the invention is especially applicable to industrial chemical processes in which sub.- stances are separated from each other by differential precipitation from a solution, and will precipitation, and that when a suitable reducing agent is added to such a solution that sulfate supersaturation is broken and the precipitation becomes normal. Normal behavior is also lestored by decreasing the acidity of the acid solution or rendering it slightly alkaline, as by the addition of sodium hydroxide to .the solution.

v This type of supersaturation is 'referred to `as stabilized because it continues for an appreciable period of time even when seed crystals are present in the solution. The excess of sulfate that is retained in solution depends upon conditions, but may be 4% or more of; the amount normally dissolvable.

Such stabilized supersaturation is found to accompany the presence inthe acid sulfate solution of a trace of a trivalent metal ion, such as ierric iron. Addition of a reducing agent to the solution in accordance'with the invention is believed to transform the ferric ions to a ferrous form, thus eliminating the ferric iron from the solution and terminating its effect. Instead of 'transforming the ferrie iron to ferrous, the same result can be obtained by removing all iron from the solution, provided the method of removal is sufficiently eiiective. However, since only a few parts per million of ferric iron may be enough to cause stabilized sulfate supersaturation, it is vusually preferable to control such supersaturation by conversion of the iron to ferrous form rather than by complete elimination of iron from the solution.

The effect can also be controlled by increasing the pH of the solution, rendering 'it'less .acid or preferably slightly alkaline. The ferric ions then become combined with hydroxyl ions and,

whether they are precipitated as ferric hydroxide or remain in solution, they lose the property of inhibiting sulfate precipitation. Conversion of the ferric ions to ferrie hydroxide can thus be considered to be a method of freeing the solution of ferric ions as such. It is indicated that much the same thing can be accomplished, that is, that the solution can be freed of ferric ions, by incorporation of the iron into an ionic compiex, for example by addition of thiocyanate to the solution. ln acid solution, irrespective of the particular acid involved, a substantial portion of the ferrie ions are free, particularly in the sense of being uncombined with hydroxyl ions, and have the effect described.

Accordingly it is possible to distinguish between two classes of states in which dissolved iron can exist in a solution which is approximately saturated with sulfate. These are a sulfate precipitation inhibiting state, represented by the presence of free ferrie ions, uncombined with hydroxyl, and occurring typically in acid solutions under oxidizingl (or at least nonreducing) conditions; and a sulfate precipitation neutral state,- represented by the absence Vof free ferric ions uncombined with hydroxyl, and occurring typically in alkaline solutions, or `in solutions under reducing conditions which transform the ferrie ions to ferrous form.

, The practical importance of this invention will be appreciated, and a more complete understandjing of its exact meaning will be gained, from the following detailed explanation of the invention as it is embodied in a typical illustrative it.' For the sake of clarity, various well known details of the fundamental process, such as means for transferring material from place to place,

are omitted from the drawing and description.

The essentials oi the process itself `are described,

for example, in United States Patent No.

In the chemical process which has been selected for purposes of illustration, sodium sulfate and boric acid are obtained by differential precipitation from a mother liquor in which sodium borate has been dissolved from a suitable ore. 'As indicated in the drawing, m-other liquor from tank l and ore containing substantially only water insoluble materials and borax '(NazBiOmlOHZO) are introduced into dissolving tank I2.` Solution of the soluble ore fraction is preferably hastened by warming the tank to about 130 F., as by steam coil I3, and by agitation of the mixture by means indicated at I4. The resulting borate solution is separated from the insoluble ore fraction in classifier and thickener i8 and in lter 2l, and the clear solution is taken to sulfate reactor 20. The solution is heated, as by steam coils 22, and sulfuric acid is added from tank 24 under control of valve 25 in sufficient quantity to convert all NazO in the solution to sodium sulfate, producing an acid solution which contains primarily sodium sulfate and boric acid. The acidified solution is heated to approximately 210 F. with the result that the concentration of sodium sulfate exceeds the normal solubility at the existing temperature and causes precipitation of anhydrous sodium sulfate. The precipitate is removed from the solution, as by centrifuge 28, and is Washed, dried and otherwise processed as may be required at 30, and delivered as substantially pure sodium sulfate at 32.

The remaining solution, saturated with sulfate and containing boric acid in high, but not quite saturating, concentration, is taken from centrifuge 28 to cooling tank 36, in which the temperature is lowered to about 105 F. by any suitable means, such as cold Water coil 3'! or evaporation of water from the solution. This increases the solubility of sulfate, which has an invertedtemperature solubility curve, so that no sulfate is precipitated. But the boric acid solubility is reduced by the lowered temperature, causing boric acid to crystallize out of solution in the cooling tank. The resulting crystals are removed, as by centrifuge 38, and boric acid is washed and dried at and delivered at 42. The remaining solution, which still contains sulfate and boric acid in suiciently high concentrations to produce saturation at 210 and 105 F. respectively, is returned as mother liquor from centrifuge 38 to storage tank l0, and is available for use in a succeeding cycle.

In the operation of the process just described, it has been found that less than the theoretical amount of sodium sulfate is sometimes precipitated in sulfate reactor 20, leading to unbalanced conditions and unsatisfactory operation of the system. In extreme cases the effective yield of sodium sulfate during a cycle is reduced to as little as 20% of the theoretical yield. This condition is corrected when the process is carried out in accordance with the present invention.

A preferred procedure for carrying out the invention in this connection is the introduction directly into the solution in sulfate reactor 20 of an appropriate quantity of sulfur dioxide gas. This can be obtained, for example, from a gas cylinder, indicated at 45, and led into the solution through line 41, the fiow of gas being regulated by valve means shown schematically at 46. Sulfur dioxide is absorbed by the solution and reacts with water to form sulfurous acid, the sulte radical acting `as a reducing agent. Alternatively, sodium sulfite can be added directly to the solution.

The presence of the reducing agent terminates the tendency toward sulfate supersaturation, and restores normal operation of the system. This is evidently brought about because the small amount of iron which is present in the solution as impurity is reduced from ferric to ferrous form. In practice iron tends to accumulate in the solution from three main sources, impurities in the borax-containing ore, impurities in the sulfuric acid, and corrosion of the apparatus. Under typical conditions, the total equilibrium concentration of iron in the mother liquor is of the order of 20 to 100 parts per million. Addition of about 3 lbs. of sulfur dioxide per 5000 gallons of solution processed is sucient to insure reduction of all ferric iron present.

Other reducing agents can be used instead of sulfite. For example finely divided zinc is effective in transforming ferric to ferrous iron and can be used to terminate sulfate supersturation caused by the presence of ferric iron. Reduction of the ferric iron can also be accomplished by electrolysis. Electrolytic reduction can be so controlled as to lead to bivalent iron, or can reduce the iron to metallic form with deposition at the electrode. In either instance the ferric iron is substantially completely removed from the solution. For convenience of description in the present specification and claims electrolytic reduction is considered to be a chemical process.

The preferred form of the invention, employing suliite as reducing agent, has the advantage that it involves the addition to the solution of no elements which are not already present, and hence does not complicate the reactions nor introduce impurities into the final products. In general the most satisfactory reducing agent must be selected for each situation in accordance with the particular chemical requirements of the process.

The reducing agent can be added to the solution either in advance of the step of precipitation, during that step, or subsequent to that step. In the present illustrative example, sulfate precipitation is brought about in part by the increase in concentration of sulfate which results from acidification of the solution, and in part by the decrease in sulfate solubility which results from heating the solution. It is broadly immaterial whether the reducing agent is added before the solution is acidied and heated, between acidiiication and heating (if these steps are performed separately) or after completion of both acidification .and heating. In the latter instance partial precipitation of the sulfate ordinarily takes place prior to the introduction of the reducing agent, reduction of the ferric iron then carrying the precipitation to its normal completion. In practice the preferred procedure is to carry out the heating, acidification and reduction substantially simultaneously and in the same vessel, sulfate reactor 20.

An alternative manner of obtaining normal sulfate precipitation is the removal from the solution of substantially all dissolved iron, or at least .of all dissolved ferric iron. In the illustrative embodiment of the invention described Iabove the dissolved borate has a tendency to hold the fer ric iron in solution during the alkaline portion of the cycle, preventing the substantially complete precipitation of ferric hydroxide which would otherwise occur. However, such precipitation can be obtained in the described process by dilution of the alkaline solution. After filtration the resulting' solution is substantially free of ferric iron and is re-concentrated, acidied and passed through the remainder of the process as before. For best results it is necessary to avoid the introduction of additional ferric iron into the solution between the step of filtration and the precipitation of sulfate. An advantage of the preferred form of the invention over the alternative form just described is the fact that contaminationof the solution with iron is rendered harmless regardless of the stage of the process at which it occurs. Thus, in a process generally analogous to the present one, but in which borate is replaced by one or more other solutes which do not prevent the precipitation of ferric hydroxide during the alkaline phase of the process, it will frequently be desirable to provide a reducing agent in the acidied solution to insure reduction of any iron which enters the solution after acidification.

`If it is preferred for any reason not to remove or transform the iron content of the solution, the effect of the ferric iron which is present vcan be minimized by carrying out the steps of acidification and sulfate precipitation in such a way that a large amount of precipitate is formed before the solution actually becomes acid. 'Ihis can be accomplished, for example by raising the temperature to approximately 210 F. before the Whole amount of required acid has Ibeen added. A portion of the acid required to transform all dissolved NazO to sulfate may be added While the solution is still cool, acidification being interrupted before the solution becomes acid. The solution is then heated, reducing the sulfate solubility-and precipitating the excess `sulfate While the ferric iron in the solution is still in the form of ferrie hydroxide. The remainder of the required acid is then added, completing transformation of the dissolved i borate and producing additional sulfate precipitation from the resulting acid solution. By this procedure any effect of uncombined ferrie ions is limited to the second relatively small portion of sulfate precipitated. Alternatively, the acid may be added continuously (but preferably slowly) rather than intermittently, the solution being heated either before or during the acidication land brought to approximately 210 F. before the solution becomes acid.

Another method of obtaining heavy precipitation while the solution is still alkaline is to bring the sulfate concentration substantially to saturation, as by addition of sodium sulfate to the solution, prior to acidification and preferably while the solution is still relatively cool. Under those conditions an appreciable quantity of additional sulfate can be dissolved. Then, when the solution is heated and acidied, sulfate saturation is attained relatively quickly, and an increased amount of sulfate is precipitated before the solution becomes acid. The total amount of sulfate precipitated is also increased by this procedure, equaling the normal (or net) -precipitate plus the amount of sulfate that was added. This increased precipitation, by a kind of mass action, appears to overcome in large part the tendency of any dissolved ferrie iron to stabilize sulfate supersaturation, with the result that the net amount of sulfate precipitated is increased and approaches the net yield that wouldbe obtained in absence of ferric ion.

The subject-matter of the two preceding paragraphs is further described and is claimed in my copending patent application, Serial No. 298,092 filed July v10, 1952, and entitledPro duction of Boric Acid and Anhydrous Sodium Sulfate, which is a continuation in part of the present application. 3

Returning now to the preferred form of the invention, in Which the ferric iron is rendered neutral toward sulfate precipitation by reduction, the ferrous iron which results from reduction is prevented from accumulating in the solution beyond a certain concentration by the 6. factth'at ah appreciable portion of thesolution is lost from the process on each cycle. Some solution is carried out at I9 with the insoluble ore fraction, and a smaller quantity is adsorbed on the precipitates which leave centrifuges 28 and 38. Under given vconditions of operation, the iron concentration in the mother liquor builds up to a value at which the amount of iron carried out of the system on each cycle equals that added to the system from such sources as have-been suggested.

In a repetitive system which continued to accumulate iron indefinitely, it would be desirable to lower the iron concentration periodically, and this could probably best be done by oxidizing the iron back to ferrie form, for example by addition of chlorine gas, and then precipitating the iron, for example With I-IzS. By such a procedure the iron concentration could be prevented from exceeding a reasonable figure. So long as the iron is kept in ferrous form, even relatively high iron concentrations do not appearl to inhibit normal precipitation of sulfate, butthey increase the possibility of iron contamination of the precipitate.

The oxidation of ferrous iron to ferri-c form, described above as a preliminary to the precipitation of iron from the solution, can also be put to positive use in a process of the type described. For example, in each cycle through which the mother liquor is passed from tankl I0 back to tank I 0, the iron content of the solution can be changed to ferrous (or to ferri-c hydroxide) form at or ahead of sulfate reactor 2l) to insure complete normal sulfate precipitation; and then changed to ferrie form (uncombined with hydroxyl) at or ahead of cooling tank 36 to inhibit precipitation of sulfate during the step of boric acid precipitation. If insufficient iron is present in the solution as natural impurity, additional iron can be introduced. Such a double transformation procedure is not ordinarily necessary in a properly designed process for the differential precipitation of substances Which have distinctly different temperature solubility curves, but it has the advantage of minimizing the possibility of sulfate contamination of the other chemical precipitated, for example, if the balance of the system should become temporarily disturbed. This advantage tends to be more important the more nearly similar are the temperature solubility curves of the materials treated.

I claim:

1. In a process in which boric acid and an anhydrous sulfate of an element selected from the class consisting of sodium and potassium are selectively precipitated from an aqueous and ordinarily acid solution containing boric acid and the sulfate as principal solutes. the solution also containing dissolved iron in sufficient concentration to stabilize supersaturation with respect to the anhydrous sulfate, the improvement which comprises treating the solution with a chemical agent selected from the class consisting of lbasic -compounds that will rea-ct with the iron to produce ferric hydroxide therefrom, and reducing agents that will react to reduce ferric ions to the ferrous state, so as to substantially so react all of the dissolved iron to prevent, by transformation of the iron to such reacted state, inhibition of precipitation of anhydrous sulfate from the solution, and precipitating the anhydrous sulfate from the solution in the presence of the iron in said reacted state, said precipitation being carried out at a temperature at aiguise@ which the :Sulfateis precipitates only i sirens forum.

i!y in. a. in Willen; @one soie enti; e trom esistita of. sotiium.- andrei. .Siusi are 1y ureoinitated nomen aqueous andere dine.. .y .oiisoiutioe Containing, torio etici one the. lorioepelsolutes is@ solutioneise eoeteiuiee dissolsesl. ironie sufeienteeueeutrae tion to. stabilise.: suoersoturationuitn issneet.- te the anhydrous soliste.. the. improvement. ooinnrises .treating thesolution situa essie that will react-Wit the iron. to ureiuoe ,.liysiroxioie therefrom. este suostentiallr soreaot alt ot the des .esi moet@ prerentty trensfotmatiou o the irouto. sueh. reaotedstete; inhibition. of. pi'eoinitatiou: oi. anhydrous sulfate `from the solution. en@ precipitating.. theaurns drousform..

3.-, In e process. in wfhiehhorie: encienden-enf hydrous sulfate of an element Selected from they Gle'sscensistne of sodium end-notessium are Seleetivelu. precipitated .from an aqueous aeidsoluf tioncontaining corio acid `andthe sulfate as prima cipal salutes. thesoluton alsoeontainins; dis-` s oivect'. iron in .suicient concentrationto stabilize suplaturation with respect. to. the. anhydrous sulfate,r the improvementwnicn comprises treat: ing the. solution with arcducing. agent that will react to reduce ferrie ions to the ferrous state, so-.as to substantia1ly'so-freact alll of the dissolved ironto. prevent, by transformation .of the.: iron to such reaotedfstate, inhibition cir-precipitation of anhydrous sulfate. from the solution,- andspreoipitating the anhydrous suiiate. from the. soiutionin the presence of the-.ironin saidferrous state, said precipitation being; carriedcut atiatemperf ateure. at.'\vi1ioh the sul-fateispreoipitatedonly inanhydrous form.

4. The improvement defined-.in claim tanduin which the. saidv reducing agent issodium sultes 5*.- The improvement-deiinedin claim 3` and in which the. said4 reducing'- agent is produced by adding sulphurk dioxide to the solution.

Gi. Ina process in which boric:` acid ananhydrousv sulfate of anelement seleoted from the f class consisting of sediumlanci. potassium arefsef lectivetyprecipitated troma-n aqueous and ordinarily acid solution containingbcric' acid.; and the sulfate as principal solutes, the so1ution-a1so containing.dissolved irenin suiicient concentration tov stabilise supers'atuijation with respect-to the ranhydrous sulfate, the iinprovementwhich comprises treat-ing the'solutio nwwitn` a reducing.

state.. sees toesoreaot en'eppreoieblo perdono! tuedissolved iron to uroduee., by' transformation of iren'toferriosstatei inhibition of; loreeipitatien of anhydrous. su. ete from. aoid. solution, and rareoipitatinaborie acid from aeid solution in .the nresenee.oitheironin the Seid; ferrie. state.,- seid heno seid` precipitation beine; carriedy out. at., a temperature, at. the. sulfate.- would. bev pre emu-stm only inenhydrous form.;

7. IIn a cyclic process for producing anhydrous Sodium sulfeteend; eerie acid.. @Solution eonteinineseoiiuru torete and also contamina dies Soltedironin-suf-eient .concentration to stsbiiize sunerseturation.witnresneette anhydrous sodium sulfate; said.-

Complisinetreetmeut et the solution .witnsuifurie .eid to. form. borie acid and sodium, Sulfate. .ureeipitetina anhydrous sodium sulfate. from thesoiutioneb e. relatively elevated temnereturei removing theI precipitata precinttatinehorie-eeidronithe. solution at.; a'relatively lower temperature., and;.removinethepreenitated torio. aei.- the. uur.revementv which. comprises treating the selutionrpriortecompletion-o tire seidsuliate precipitation With-.a reducing agent tua-twill react tofrednoe. ferrieions to the ferrous state,4 so. esto, sulots.entie11s.x so, reaetnaliof the Solvefiiron to.z prevent; br'transiormation Qfcths iron to Suo11;,reacte.c;i;state,. inhibition. or lorefivireutionA oiy anhydroussul-'etezfrem the' selntiom and completi-ne thepreoipitst-ion et anhydrous-sodium Sulfate. fromthe; solution thefpresenee et. the iron in saiob ferrous; state.. saisi-.sulfatefpreeinitsw tion being-carried-out1at a temperatura @which the sulfate is precipitated only' in. anhydrous form.

a.A The. improvement, defined irrolaim Y.; endfin which .the said yreductie agentia sodium sulter 9i The. improvement deinedin CIaimfLzanAin which theosaictzrediucing.: agent; is produced.. by' adding sulphur dioxide. to. the solution@ 19. Thel -improszernent.-zdefined:r in claim: and including( treating the solution, after thev sii-eppoi sulfate precipitation prior to;l completion' of tine-step of i bone-acid precipitation, with an oxis dizingx agent reactto oindizeiierfrous ions t-o the-ferrie.` state, so as tolsoireactat leastzan-ap: preciable prtionoi :the dissolveda iron to produce, by: transformation .of iron to, feuric stattei .inhibi-v tion @precipitation ot'anhydrous sodium sulfate from acid; solution, and; comp1eting the. precipita. tiono'fboricfacidffrom lacidi solution in the: prese ence offthe, iron in thersaidz ier'ric'statefseid bon-.ic acid precipitation. beingcarried out` at a temper, atureeat which. the sulfate/W011i@ be; precipitated oniyinlanhyd-rousxform.

DONALD. Si TAY-LGR.

References, the. .filer Qf'vfthisipaw Tzil;l STATES' PATENTS Number Name.. Date; IMQ-6.3- Gilmn 1350.195 Frenk 255189.95534.' J 3.99121 Alle,... 1.0.-., 19.37 

1. IN A PROCESS IN WHICH BORIC ACID AND AN ANHYDROUS SULFATE OF AN ELEMENT SELECTED FROM THE CLASS CONSISTING OF SODIUM AND POTASSIUM ARE SELECTIVELY PRECIPITATED FROM AN AQUEOUS AND ORDINARILY ACID SOLUTION CONTAINING BORIC ACID AND THE SULFATE AS PRINCIPAL SOLUTES, THE SOLUTION ALSO CONTAINING DISSOLVED IRON IN SUFFICIENT CONCENTRATION TO STABILIZE SUPERSATURATION WITH RESPECT TO THE ANHYDROUS SULFATE, THE IMPROVEMENT WHICH COMPRISES TREATING THE SOLUTION WITH A CHEMCAL AGENT SELECTED FROM THE CLASS CONSISTING OF BASIC COMPOUND THAT WILL REACT WITH THE IRON TO PRODUCE FERRIC HYDROXIDE THEREFROM, AND REDUCING AGENTS THAT WILL REACT TO REDUCE FERRIC IONS TO THE FERROUS STATE, SO AS TO SUBSTANTIALLY SO REACT ALL OF THE DISSOLVED IRON TO PREVENT, BY TRANSFORMATION OF THE IRON TO SUCH REACTED STATE, INHIBITION OF PRECIPITATION OF ANHYDROUS SULFATE FROM THE SOLUTION, AND PRECIPITATION THE ANHYDROUS SULFATE FROM THE SOLUTION IN THE PRESENCE OF THE IRON IN SAID REACTED STATE, SAID PRECIPITATION BEING CARRIED OUT AT A TEMPERATURE AT WHICH THE SULFATE IS PRECIPITATED ONLY IN ANHYDROUS FORM. 